No idea why; the plugin code is pretty straight-forward and uses the built-in WP hooks and functions as much as possible.
You're saying this is an intermittent error, so I probably can't reproduce it... What happens if all your other plugins are disabled, you are using the built-in theme (twenty-twelve? twenty-thirteen?) and this is the only plugin running?
Please enable WP_DEBUG and error messages by adding the following to your wp-config file:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_DISPLAY', true );
Then deactivate all your other plugins and switch to a wp theme (twenty-twelve or twenty-thirteen) so we can also check if it's an issue with your theme.
If you don't see any errors with that set-up, it's not a problem with the plugin itself, but may be something that only happens in combination with your theme or other plugins. If that's the case, the next step is to reactivate your theme and check again, then your plugins one-by-one, until you can identify what combination triggers the error.
When you see the error again, stop, and send me the exact error message including error code that your server is throwing, what operating system your server uses, what error messages, if any, WP_DEBUG is showing, your WP version, a complete list of active plugins, and the last thing you activated before you started seeing the error again.